Chapter 2 -Biological classification
Since the dawn of civilisation, there have been many attempts to classify
living organisms. It was done instinctively not using criteria that were
scientific but borne out of a need to use organisms for our own use -
for food, shelter and clothing. Aristotle was the earliest to attempt a more
scientific basis for classification. He used simple morphological characters
to classify plants into trees, shrubs and herbs. He also divided animals
into two groups, those which had red blood and those that did not.
In Linnaeus' time a Two Kingdom system of classification with
Plantae and Animalia kingdoms was developed that included all
plants and animals respectively. This system did not distinguish between
the eukaryotes and prokaryotes, unicellular and multicellular organisms
and photosynthetic (green algae) and non-photosynthetic (fungi)
organisms. Classification of organisms into plants and animals was easily
done and was easy to understand, but, a large number of organisms
did not fall into either category. Hence the two kingdom classification
used for a long time was found inadequate. Besides, gross morphology
a need was also felt for including other characteristics like cell structure,
nature of wall, mode of nutrition, habitat, methods of reproduction,
evolutionary relationships, etc. Classification systems for the living
organisms have hence, undergone several changes over the time.
Though plant and animal kingdoms have been a constant under all
different systems, the understanding of what groups/organisms be
included under these kingdoms have been changing; the number and
nature of other kingdoms have also been understood differently by
different scientists over the time.
R.H. Whittaker (1969) proposed a Five Kingdom Classification.
The
kingdoms defined by him were named Monera, Protista, Fungi, Plantae
and Animalia.
The main criteria for classification used by him include cell
structure, body organisation, mode of nutrition, reproduction and
phylogenetic relationships.
The three-domain system has also been proposed that divides the Kingdom
Monera into two domains, leaving the remaining eukaryotic kingdoms in the
third domain and thereby a six kingdom classification.
Earlier classification systems included bacteria, blue green algae, fungi, mosses,
ferns, gymnosperms and the angiosperms under -Plants.
The character
that unified this whole kingdom was that all the organisms included had a
cell wall in their cells.
This placed together groups which widely differed in
other characteristics. It brought together the prokaryotic bacteria and the
blue green algae ( cyanobacteria) with other groups which were eukaryotic.
It also grouped together the unicellular organisms and the multicellular
ones, say, for example, Chlamydomonas and Spirogyra were placed together
under algae.
The classification did not differentiate between the heterotrophic
group - fungi, and the autotrophic green plants, though they also showed
a characteristic difference in their walls composition – the fungi had chitin
in their walls while the green plants had a cellulosic cell wall.
When such
characteristics were considered, the fungi were placed in a separate
kingdom - Kingdom Fungi.
All prokaryotic organisms were grouped
together under Kingdom Monera and the unicellular eukaryotic organisms
were placed in Kingdom Protista.
Kingdom Protista has brought together
Chlamydomonas, Chlorella (earlier placed in Algae within Plants and both
having cell walls) with Paramoecium and Amoeba (which were earlier placed
in the animal kingdom which lack cell wall).
It has put together organisms
which, in earlier classifications, were placed in different kingdoms. This
happened because the criteria for classification changed.
This kind of
changes will take place in future too depending on the improvement in our
understanding of characteristics and evolutionary relationships.
Over time,
an attempt has been made to evolve a classification system which reflects
not only the morphological, physiological and reproductive similarities,
but is also phylogenetic, i.e., is based on evolutionary relationships.
In this chapter we will study characteristics of Kingdoms Monera,
Figure Bacteria of different shapes
Kingdom monera
Bacteria are the sole members of the Kingdom Monera.
They are the most
abundant micro-organisms. Bacteria occur almost everywhere. Hundreds
of bacteria are present in a handful of soil. They also live in extreme habitats
such as hot springs, deserts, snow and deep oceans where very few other
life forms can survive. Many of them live in or on other organisms as
parasites.
Bacteria are grouped under four categories based on their shape:
The spherical Coccus (pl.: cocci),
the rod-shaped Bacillus (pl.: bacilli),
the
comma-shaped Vibrium (pl.: vibrio) and the spiral Spirillum (pl.: spirilla)
Some of the bacteria are
autotrophic, i.e., they synthesise their own food from inorganic substrates.
They may be photosynthetic autotrophic or chemosynthetic autotrophic.
The vast majority of bacteria are heterotrophs, i.e., they depend on other
organisms or on dead organic matter for food.
Archaebacteria
These bacteria are special since they live in some of the most harsh habitats
such as extreme salty areas (halophiles), hot springs (thermoacidophiles)
and marshy areas (methanogens).
Archaebacteria differ from other bacteria
in having a different cell wall structure and this feature is responsible for
their survival in extreme conditions.
Methanogens are present in the gut
of several ruminant animals such as cows and buffaloes and they are
responsible for the production of methane (biogas) from the dung of these
animals.
Figure filamentous blue-green algae - Nostoc
Eubacteria
There are thousands of different eubacteria or ‘true bacteria’.
They are characterised by the presence of a
rigid cell wall, and if motile, a flagellum.
The
cyanobacteria (also referred to as blue-green algae)
have chlorophyll a similar to green plants and are
photosynthetic autotrophs (Figure).
The
cyanobacteria are unicellular, colonial or filamentous,
freshwater/marine or terrestrial algae.
The colonies
are generally surrounded by gelatinous sheath.
They
often form blooms in polluted water bodies.
Some of
these organisms can fix atmospheric nitrogen in
specialised cells called heterocysts, e.g., Nostoc and
Anabaena.
Chemosynthetic autotrophic bacteria
oxidise various inorganic substances such as
nitrates, nitrites and ammonia and use the released
energy for their ATP production. They play a great role
in recycling nutrients like nitrogen, phosphorous,
iron and sulphur.
Heterotrophic bacteria are most abundant in
nature.
The majority are important decomposers.
They are helpful in making curd from milk,
production of antibiotics, fixing nitrogen in legume
roots, etc.
Some are pathogens causing damage
to human beings, crops, farm animals and pets.
Cholera, typhoid, tetanus, citrus canker are well
known diseases caused by different bacteria.
Bacteria reproduce mainly by fission (Figure). Sometimes, under unfavourable conditions,
they produce spores.
They also reproduce by a
sort of sexual reproduction by adopting a
primitive type of DNA transfer from one bacterium
to the other.
The Mycoplasma are organisms that
completely lack a cell wall.
They are the smallest
living cells known and can survive without oxygen. Many mycoplasma
are pathogenic in animals and plants.
1. How did Aristotle try to classify plants?
2. What was animal classification as suggested by Aristotle?
3. What was two Kingdom system of classification during the time of Linnaeus?
4. How prokaryotes and eukaryotes are distinguished?
5. Why the two kingdom classification was found to be inadequate?
6. In which year Whittaker proposed his classification?
7. Which are 5 kingdoms as proposed in the 5 kingdom classification by Whittaker?
8. Write four criteria which were adopted in five kingdom classification?
9. Explain three domain system.
10. How many kingdoms are there in three domain system.
11. Which all groups were put under the kingdom plantae in two Kingdom classification?
12. Name the character which brought together prokaryotic bacteria and blue green algae in the kingdom plantae.
13. Which is the other name of cyanobacteria?
14. How Chlamydomonas and Spirogyra differ in there structure?
15. What type of Nutrition green plants have?
15. Name the nutrition which we find in fungi.
16. How fungi and green plants differ in there cell wall composition?
17. Which groups were placed in the Kingdom Monera?
18. In which Kingdom unicellular eukaryotic organisms are placed?
19. How do you understand the term phylogenetic?
20. Why the classification system has changed over a period time?
21. Who are the sole members of Kingdom Monera?
22. Name four extreme habitats of the organisms belonging to Kingdom Monera.
23. Which are four categories of bacteria based on their shape?
24. Which are two categories of autotrophic bacteria?
25. What do you mean by heterotrophic bacteria?
26. What are halophiles?
27. What are thermoacidophiles?
28. What are methanogens?
29. Which feature is responsible for the survival of Archaebacteria under adverse conditions?
30. Name two ruminants (animals).
31. Which gas is also known as biogas?
32. What are heterocysts?
33. Give two examples of eubacteria having heterocysts.
34. Name various substances Chemosynthetic autotrophic bacteria oxidise?
35. Which molecule is produced from the energy released when Chemosynthetic autotrophic bacteria oxidize?
36. Give at least three examples in which Heterotrophic bacteria are helpful.
37. Name four diseases caused by different bacteria.
38. How bacteria reproduce under unfavourable conditions?
39. Which are smallest living cells known who can survive without oxygen?
40. Mycoplasma do not have a cell wall and are not pathogenic.
Comment on this statement.
Kingdom protista
Extract41-
All single-celled eukaryotes are placed under Protista, but the boundaries
of this kingdom are not well defined.
What may be - a photosynthetic
protistan to one biologist may be -a plant to another.
In this book we
include Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds and
Protozoans under Protista.
Members of Protista are primarily aquatic.
This kingdom forms a link with the others dealing with plants, animals
and fungi. Being eukaryotes, the protistan cell body contains a well defined
nucleus and other membrane-bound organelles. Some have flagella or
cilia. Protists reproduce asexually and sexually by a process involving
cell fusion and zygote formation.
Chrysophytes
This group includes diatoms and golden algae (desmids).
They are found
in fresh water as well as in marine environments.
They are microscopic
and float passively in water currents (plankton).
Most of them are
photosynthetic.
In diatoms the cell walls form two thin overlapping shells,
which fit together as in a soap box. The walls are embedded with silica
and thus the walls are indestructible. Thus, diatoms have left behind
large amount of cell wall deposits in their habitat; this accumulation over
billions of years is referred to as - diatomaceous earth. Being gritty this
soil is used in polishing, filtration of oils and syrups. Diatoms are the
chief -producers in the oceans.
Dinoflagellates
These organisms are mostly marine and photosynthetic.
They appear yellow, green, brown, blue or red depending
on the main pigments present in their cells.
The cell wall
has stiff cellulose plates on the outer surface.
Most of
them have two flagella; one lies longitudinally and the
other transversely in a furrow between the wall plates.
Very often, red dinoflagellates (Example: Gonyaulax)
undergo such rapid multiplication that they make the
sea appear red (red tides). Toxins released by such large
numbers may even kill other marine animals such as
fishes.
Euglenoids
Majority of them are fresh water organisms found in
stagnant water. Instead of a cell wall, they have a protein
rich layer called pellicle which makes their body flexible.
They have two flagella, a short and a long one. Though
they are photosynthetic in the presence of sunlight, when
deprived of sunlight they behave like heterotrophs by
predating on other smaller organisms. Interestingly, the
pigments of euglenoids are identical to those present in
higher plants. Example: Euglena (Figure ).
Slime moulds are saprophytic protists.
The fungi constitute a unique kingdom of heterotrophic organisms. They
show a great diversity in morphology and habitat. You must have seen
fungi on a moist bread and rotten fruits. The common mushroom you eat
and toadstools are also fungi. White spots seen on mustard leaves are due
to a parasitic fungus. Some unicellular fungi, e.g., yeast are used to make
bread and beer. Other fungi cause diseases in plants and animals; wheat
rust-causing Puccinia is an important example. Some are the source of
antibiotics, e.g., Penicillium. Fungi are cosmopolitan and occur in air, water,
soil and on animals and plants. They prefer to grow in warm and humid
places. Have you ever wondered why we keep food in the refrigerator ? Yes,
it is to prevent food from going bad due to bacterial or fungal infections.
With the exception of yeasts which are unicellular, fungi are
filamentous. Their bodies consist of long, slender thread-like structures
called hyphae. The network of hyphae is known as mycelium. Some hyphae
are continuous tubes filled with multinucleated cytoplasm - these are
called coenocytic hyphae. Others have septae or cross walls in their
hyphae. The cell walls of fungi are composed of chitin and polysaccharides.
Most fungi are heterotrophic and absorb soluble organic matter from
dead substrates and hence are called saprophytes.
Phycomycetes
Ascomycetes
Commonly known as sac-fungi, the ascomycetes are mostly
multicellular, e.g., Penicillium, or rarely unicellular, e.g., yeast
(Saccharomyces). They are saprophytic, decomposers,
parasitic or coprophilous (growing on dung).
The fruiting body is known as ascocarp. There are four types of ascocarps:
Cleistothecium- The fruiting body is spherical and remains tightly closed, e.g. Aspergillus Commonly known forms of basidiomycetes are mushrooms, bracket fungi
or puffballs. Deuteromycetes
Commonly known as imperfect fungi because only the asexual or
vegetative phases of these fungi are known. When the sexual forms of
these fungi were discovered they were moved into classes they rightly
belong to. It is also possible that the asexual and vegetative stage have
been given one name (and placed under deuteromycetes) and the sexual
stage another (and placed under another class). Later when the linkages
were established, the fungi were correctly identified and moved out of
deuteromycetes. Once perfect (sexual) stages of members of
dueteromycetes were discovered they were often moved to ascomycetes
and basidiomycetes. The deuteromycetes reproduce only by asexual spores
known as conidia. The mycelium is septate and branched. Some members
are saprophytes or parasites while a large number of them are
decomposers of litter and help in mineral cycling. Some examples are
Alternaria, Colletotrichum and Trichoderma.
Extract41-50
Viruses, Viroids, Prions and lichens
The classification systems have undergone several changes with time. The first
attempt of classification was made by Aristotle. He classified plants as herbs, shrubs, and
trees. Animals, on the other hand, were classified on the basis of presence or absence of red
blood cells. This system of classification failed to classify all the known organisms.
Therefore, Linnaeus gave a two-kingdom system of classification. It consists of kingdom
Plantae and kingdom Animalia. However, this system did not differentiate between unicellular
and multicellular organisms and between eukaryotes and prokaryotes. Therefore, there were
large numbers of organisms that could not be classified under the two kingdoms.
To solve these problems, a five kingdom system of classification was proposed by R.H
Whittaker in 1969. On the basis of characteristics, such as cell structure, mode of nutrition,
presence of cell wall, etc., five kingdoms, Monera, Protista, Fungi, Plantae, and Animalia were
formed.
2. State two economically important uses of:
(a) Heterotrophic bacteria-
3. What is the nature of cell-walls in diatoms? Diatoms:
4. Find out what do the terms - algal bloom and red-tides signify.
Algal bloom
5. How are viroids different from viruses?
6. Describe briefly the four major groups of Protozoa.
7. Plants are autotrophic. Can you think of some plants that are partially
heterotrophic?
Plants have autotrophic mode of nutrition as they contain chlorophyll pigment. Thus,
they have the ability to prepare their own food by the process of photosynthesis. However,
some insectivorous plants are partially heterotrophic. They have various means of capturing
insects so as to supplement their diet with required nutrients derived from insects, causing
proliferation of growth. The examples include pitcher plant (Nepenthes), Venus fly trap,
bladderwort, and sundew plant.
8. What do the terms phycobiont and mycobiont signify?
Phycobiont refers to the algal component of the lichens and mycobiont refers to the
fungal component. Algae contain chlorophyll and prepare food for fungi whereas the fungus
provides shelter to algae and absorbs water and nutrients from the soil. This type of
relationship is referred to as symbiotic.
9. Give a comparative account of the classes of Kingdom Fungi under the following:
Phycomycetes - This group of fungi includes members such as Rhizopus, Albugo, etc.
10. What are the characteristic features of Euglenoids?
Some characteristic features of Euglenoids are as follows.
11. Give a brief account of viruses with respect to their structure and nature of
genetic material. Also name four common viral diseases.
Viruses are sub-microscopic infectious agents that can infect all living organisms. A
virus consists of genetic material surrounded by a protein coat. The genetic material may be
present in the form of DNA or RNA.
12. Organise a discussion in your class on the topic -Are viruses living or nonliving? Viruses are microscopic organisms that have characteristics of both living and nonliving.
E-tutorials
Fungi
The structure of fungi -
a. filamentous structure except the yeast cells, single-celled or multicellular organisms.
b. Fungi have long thread-like structures known as hyphae. These hyphae together form mycelium. A mycelium is a network of fungal threads or hyphae.
c. Fungi possess a cell wall which is made up of chitin and polysaccharides.
Characteristics of Fungi
Following are the important characteristics of fungi:
Fungi are eukaryotic, non-vascular, non-motile and heterotrophic organisms.
They may be unicellular or filamentous.
They reproduce by means of spores.
Fungi exhibit the phenomenon of alternation of generation.
Fungi lack chlorophyll and hence cannot perform photosynthesis.
Fungi store their food in the form of starch.
Biosynthesis of chitin occurs in fungi.
The nuclei of the fungi are very small.
The fungi have no embryonic stage. They develop from the spores.
The mode of reproduction is sexual or asexual.
Some fungi are parasitic and can infect the host.
Fungi produce a chemical called pheromone which leads to sexual reproduction in fungi.
Examples include mushrooms, moulds and yeast.
Classification of Fungi
Kingdom Fungi are classified based on different modes. The different classification of fungi is as follows:
Based on Mode of nutrition
On the basis of nutrition, kingdom fungi can be classified into 3 groups.
Saprophytic – The fungi obtain their nutrition by feeding on dead organic substances. Examples: Rhizopus, Penicillium and Aspergillus.
Parasitic – The fungi obtain their nutrition by living on other living organisms (plants or animals) and absorb nutrients from their host. Examples: Taphrina and Puccinia.
Symbiotic – These fungi live by having an interdependent relationship with other species in which both are mutually benefited. Examples: Lichens and mycorrhiza. Lichens are the symbiotic association between algae and fungi. Here both algae and fungi are mutually benefited as fungi provide shelter for algae and in reverse algae synthesis carbohydrates for fungi. Mycorrhiza is the symbiotic association present between fungi and plants. Fungi improve nutrient uptake by plants, whereas, plants provides organic molecules like sugar to the fungus.
Based on Spore Formation
Kingdom Fungi are classified into the following based on the formation of spores:
Zygomycetes – These are formed by the fusion of two different cells. The sexual spores are known as zygospores, while the asexual spores are known as sporangiospores. The hyphae are without the septa. Example – Mucor.
Ascomycetes – They are also called sac fungi. They can be coprophilous, decomposers, parasitic or saprophytic. The sexual spores are called ascospores. Asexual reproduction occurs by conidiospores. Example – Saccharomyces.
Basidiomycetes – Mushrooms are the most commonly found basidiomycetes and mostly live as parasites. Sexual reproduction occurs by basidiospores. Asexual reproduction occurs by conidia, budding or fragmentation. Example- Agaricus.
Deuteromycetes – They are otherwise called imperfect fungi as they do not follow the regular reproduction cycle as the other fungi. They do not reproduce sexually. Asexual reproduction occurs by conidia. Example – Trichoderma.
Also Read: Difference Between Algae And Fungi
How many types of bacteria are there? (Classify Bacteria)
Classification
On the basis of their shapes
These bacteria can grow in chains, clusters or pairs.
On the basis of their cell wall
i. Gram-positive bacteria.
The body moves
along decaying twigs and leaves engulfing organic
material. Under suitable conditions, they form an
aggregation called plasmodium which may grow and
spread over several feet. During unfavourable conditions,
the plasmodium differentiates and forms fruiting bodies
bearing spores at their tips. The spores possess true walls.
They are extremely resistant and survive for many years,
even under adverse conditions. The spores are dispersed
by air currents.
Protozoans
All protozoans are heterotrophs and live as predators or
parasites. They are believed to be primitive relatives of
animals. There are four major groups of protozoans.
Amoeboid protozoans: These organisms live in fresh
water, sea water or moist soil. They move and capture
their prey by putting out pseudopodia (false feet) as in Amoeba. Marine
forms have silica shells on their surface. Some of them such as Entamoeba
are parasites.
Flagellated protozoans: The members of this group are either free-living
or parasitic. They have flagella. The parasitic forms cause diaseases such
as sleeping sickness. Example: Trypanosoma.
Ciliated protozoans: These are aquatic, actively moving organisms because
of the presence of thousands of cilia. They have a cavity (gullet) that opens
to the outside of the cell surface. The coordinated movement of rows of
cilia causes the water laden with food to be steered into the gullet. Example:
Paramoecium (Figure 2.4b).
Sporozoans: This includes diverse organisms that have an infectious
spore-like stage in their life cycle. The most notorious is Plasmodium
(malarial parasite) which causes malaria, a disease which has a staggering
effect on human population.
→
Fungi
↔
Extract31-40
Kingdom fungi
Those that depend
on living plants and animals are called parasites.
They can also live as
symbionts - in association with algae as lichens and with roots of higher
plants as mycorrhiza.
Reproduction in fungi can take place by vegetative means -
fragmentation, fission and budding. Asexual reproduction is by spores
called conidia or sporangiospores or zoospores, and sexual reproduction
is by oospores, ascospores and basidiospores.
The various spores are
produced in distinct structures called fruiting bodies.
The sexual cycle
involves the following three steps:
(i) Fusion of protoplasms between two motile or non-motile gametes
called plasmogamy.
(ii) Fusion of two nuclei called karyogamy.
(iii) Meiosis in zygote resulting in haploid spores.
When a fungus reproduces sexually, two haploid
hyphae of compatible mating types come together and
fuse. In some fungi the fusion of two haploid cells
immediately results in diploid cells (2n).
However, in other
fungi (ascomycetes and basidiomycetes), an intervening
dikaryotic stage (n + n, i.e., two nuclei per cell) occurs;
such a condition is called a dikaryon and the phase is
called dikaryophase of fungus.
Later, the parental nuclei
fuse and the cells become diploid. The fungi form fruiting
bodies in which reduction division occurs, leading to
formation of haploid spores.
The morphology of the mycelium, mode of spore
formation and fruiting bodies form the basis for the
division of the kingdom into various classes.
Ascomycetes
Basidiomycetes and Deuteromycetes
Commonly known as imperfect fungi
Members of phycomycetes are found in aquatic habitats
and on decaying wood in moist and damp places or as
obligate parasites on plants. The mycelium is aseptate
and coenocytic. Asexual reproduction takes place by
zoospores (motile) or by aplanospores (non-motile). These
spores are endogenously produced in sporangium.
A
zygospore is formed by fusion of two gametes. These
gametes are similar in morphology (isogamous) or
dissimilar (anisogamous or oogamous). Some common
examples are Mucor (Figure 2.5a), Rhizopus (the bread
mould mentioned earlier) and Albugo (the parasitic fungi
on mustard).
Mycelium
is branched and septate. The asexual spores are conidia produced
exogenously on the special mycelium called conidiophores. Conidia on
germination produce mycelium.
Sexual spores are called ascospores
which are produced endogenously in sac like asci (singular ascus). These
asci are arranged in different types of fruiting bodies called ascocarps.
→
Perithecium- The fruiting body is flask-shaped with one external opening, e.g. Neurospora
Apothecium- The fruiting body is cup-shaped and asci are present in hymenium, e.g. Peziza
Ascostroma- There is no differentiated fruiting body. Asci are present in the stroma, e.g. Mycosphaerella
Some examples are Aspergillus (Figure ), Claviceps and Neurospora.
Neurospora is used extensively in biochemical and genetic work. Many
members like morels and truffles are edible and are considered delicacies.
Basidiomycetes
They grow in soil, on logs and tree stumps and in living
plant bodies as parasites, e.g., rusts and smuts. The mycelium is branched
and septate. The asexual spores are generally not found, but vegetative
reproduction by fragmentation is common. The sex organs are absent,
but plasmogamy is brought about by fusion of two vegetative or somatic
cells of different strains or genotypes.
The resultant structure is dikaryotic
which ultimately gives rise to basidium. Karyogamy and meiosis take
place in the basidium producing four basidiospores. The basidiospores
are exogenously produced on the basidium (pl.: basidia). The basidia are
arranged in fruiting bodies called basidiocarps. Some common members
are Agaricus (mushroom) (Figure ), Ustilago (smut) and Puccinia (rust fungus).
Kingdom plantae
Kingdom Plantae includes all eukaryotic chlorophyll-containing
organisms commonly called plants. A few members are partially
heterotrophic such as the insectivorous plants or parasites. Bladderwort
and Venus fly trap are examples of insectivorous plants and Cuscuta is a
parasite. The plant cells have an eukaryotic structure with prominent
chloroplasts and cell wall mainly made of cellulose. You will study the
eukaryotic cell structure in detail in Chapter 8.
Plantae includes algae,
bryophytes, pteridophytes, gymnosperms and angiosperms.
Life cycle of plants has two distinct phases - the diploid sporophytic
and the haploid gametophytic - that alternate with each other.
The lengths
of the haploid and diploid phases, and whether these phases are free -
living or dependent on others, vary among different groups in plants.
This phenomenon is called alternation of generation. You will study
further details of this kingdom in Chapter 3.
Kingdom Animalia
This kingdom is characterised by heterotrophic eukaryotic organisms
that are multicellular and their cells lack cell walls. They directly or
indirectly depend on plants for food. They digest their food in an internal
cavity and store food reserves as glycogen or fat. Their mode of nutrition
is holozoic -by ingestion of food. They follow a definite growth pattern
and grow into adults that have a definite shape and size. Higher forms
show elaborate sensory and neuromotor mechanism. Most of them are
capable of locomotion.
The sexual reproduction is by copulation of male and female followed
by embryological development. Salient features of various phyla are
described in Chapter 4.
All of us who have suffered the ill effects of common cold or ‘flu’ know
what effects viruses can have on us, even if we do not associate it with our
condition.
Viruses did not find a place in classification since they are not
considered truly ‘living’, if we understand living as those organisms that
have a cell structure. The viruses are non-cellular organisms that are
characterised by having an inert crystalline structure outside the living cell.
Once they infect a cell they take over the machinery of the host cell to replicate
themselves, killing the host. Would you call viruses living or non-living?
The name virus that means venom or poisonous fluid was given by
Pasteur. D.J. Ivanowsky (1892) recognised certain microbes as causal
organism of the mosaic disease of tobacco (Figure 2.6a).
These were found to be smaller than bacteria because they passed through bacteria-proof filters. M.W. Beijerinek (1898) demonstrated that the extract of the infected
plants of tobacco could cause infection in healthy plants and called the
fluid as Contagium vivum fluidum (infectious living fluid). W.M. Stanley
(1935) showed that viruses could be crystallised and crystals consist
largely of proteins. They are inert outside their specific host cell. Viruses
are obligate parasites.
In addition to proteins, viruses also contain genetic material, that could
be either RNA or DNA. No virus contains both RNA and DNA.
A virus is
a nucleoprotein and the genetic material is infectious. In general, viruses
that infect plants have single stranded RNA and viruses that infect animals
have either single or double stranded RNA or double stranded DNA.
Bacterial viruses or bacteriophages (viruses that infect the bacteria) are
usually double stranded DNA viruses (Figure 2.6b). The protein coat
called capsid made of small subunits called capsomeres, protects the
nucleic acid. These capsomeres are arranged in helical or polyhedral
geometric forms. Viruses cause diseases like mumps, small pox, herpes
and influenza. AIDS in humans is also caused by a virus. In plants, the
symptoms can be mosaic formation, leaf rolling and curling, yellowing
and vein clearing, dwarfing and stunted growth.
Viroids : In 1971, T.O. Diener discovered a new infectious agent that
was smaller than viruses and caused potato spindle tuber disease. It was
found to be a free RNA; it lacked the protein coat that is found in viruses,
hence the name viroid. The RNA of the viroid was of low molecular weight.
Prions : In modern medicine certain infectious neurological diseases
were found to be transmitted by an agent consisting of abnormally folded
protein. The agent was similar in size to viruses. These agents were called
prions. The most notable diseases caused by prions are bovine spongiform
encephalopathy (BSE) commonly called mad cow disease in cattle and
its analogous variant Cr–Jacob disease (CJD) in humans.
Lichens : Lichens are symbiotic associations i.e. mutually useful
associations, between algae and fungi. The algal component is known as
phycobiont and fungal component as mycobiont, which are autotrophic
and heterotrophic, respectively. Algae prepare food for fungi and fungi
provide shelter and absorb mineral nutrients and water for its partner.
So close is their association that if one saw a lichen in nature one would
never imagine that they had two different organisms within them.
Lichens are very good pollution indicators - they do not grow in polluted areas.
Exercise
1. Discuss how classification systems have undergone several changes over a
period of time?
(a) heterotrophic bacteria
(b) archaebacteria
They act as decomposers and help in the formation of humus.
They help in the production of curd from milk.
Many antibiotics are obtained from some species of bacteria.
Many soil bacteria help in fixation of atmospheric nitrogen.
(b) Archaebacteria
(1) Methane gas is produced from the dung of ruminants by the methanogens.
(2) Methanogens are also involved in the formation of biogas and sewage treatmen
They are the only organisms with cell walls made of transparent, opaline silica.
Diatoms are unicellular photosynthetic algae that produce a silica exoskeleton (frustule) which exposes a highly ordered nano to micro scale morphology.
This cell wall is rich in silica and is porous. A Silicated cell wall of diatoms is called frustule.
i. Algal bloom is an increase in the population of algae or blue-green algae in water.
ii. This results in discoloration of the water body.
iii. Due to above phenomenon an increase in the biological oxygen demand (BOD) takes place.
iv. As adequate amount of oxygen is not available in the water body, the death of fishes and other aquatic animals takes place.
Red-tides
a. Red tides are caused by red dinoflagellates (Gonyaulax).
b. They multiply very fast.
c. Due to their large numbers, the sea appears red in color.
d. A large amounts of toxins is released in the water during their multiplication which is the cause of the death of a large number of fish.
(i) mode of nutrition
(ii) mode of reproduction
(i) Mode of nutrition
They are obligate parasites on plants or are found on decaying matter such as wood.
(ii) Mode of reproduction
Asexual reproduction takes place through motile zoospores or non-motile aplanospores that are
produced endogenously in sporangium.
Sexual reproduction may be of isogamous, anisogamous, or oogamous type. It results in the
formation of thick-walled zygospore.
Ascomycetes- This group of fungi includes members such as Penicillium,
Aspergillus, Claviceps, and Neurospora.
(i) Mode of nutrition -
They are sporophytic, decomposers, parasitic or coprophilous (growing on dung).
(ii) Mode of reproduction -
Asexual reproduction occurs through asexual spores produced exogenously, such as conidia
produced on conidiophores.
Sexual reproduction takes place through ascospores produced endogenously in sac-like asci
and arranged inside ascocarps.
Basidiomycetes- This group of fungi includes members such as Ustilago, Agaricus and Puccinia.
(i) Mode of nutrition -
They grow as decomposers in soil or on logs and tree stumps. They also occur as parasites in
plants causing diseases such as rusts and smuts.
(ii) Mode of reproduction -
Asexual reproduction takes place commonly through fragmentation. Asexual spores are absent.
Sex organs are absent but sexual reproduction takes place through plasmogamy. It involves
fusion of two different strains of hyphae. The resulting dikaryon gives rise to a basidium. Four
basidiospores are produced inside a basidium.
Deuteromycetes - This group of fungi includes members such as Alternaria, Trichoderma, and
Colletotrichum.
(i) Mode of nutrition
Some members are saprophytes while others are parasites. However, a large number act as
decomposers of leaf litter.
(ii) Mode of reproduction
Asexual reproduction is the only way of reproduction in deuteromycetes. It occurs through
asexual spores called conidia.
Sexual reproduction is absent in deuteromycetes.
Euglenoids (such as Euglena) are unicellular protists commonly found in fresh water.
Instead of cell wall, a protein-rich cell membrane known as pellicle is present.
They bear two flagella on the anterior end of the body.
A small light sensitive eye spot is present.
They contain photosynthetic pigments such as chlorophyll and can thus prepare their own food.
However, in absence of light, they behave similar to heterotrophs by capturing other small
aquatic organisms.
They have both plant and animal-like features, which makes them difficult to classify.
Most of the viruses, infecting plants, have single stranded RNA as genetic material. On the
other hand, the viruses infecting animals have single or double stranded RNA or double
stranded DNA.
Bacteriophages or viruses infecting bacteria mostly have double stranded DNA. Their protein
coat called capsid is made up of capsomere subunits. These capsomeres are arranged in helical
or polyhedral geometric forms.
A.I.D.S, small pox, mumps, and influenza are some common examples of viral diseases.
A virus consists of a strand of DNA or RNA covered by a protein coat. This presence of
nucleic acid (DNA or RNA) suggests that viruses are alive. In addition, they can also respond to
their environment (inside the host cell) in a limited manner. However, some other characters,
such as their inability to reproduce without using the host cell machinery and their acellular
nature, indicate that viruses are non-living. Therefore, classifying viruses has remained a
mystery for modern systematics.
i. Spherical shaped bacteria also called the cocci.
i. Monococci - single cocci
ii. Diplococci -cocci arranged in pairs. e.g., Neisseria gonorrhoeae
iii. Streptococci - cocci arranged in chains. e.g., Streptococcus mutans
iv.Staphylococci - clusters of cocci. e.g., Staphylococcus aureus
v. Tetrads- cocci in groups of four. e.g., Pediococcus
vi. Sarcina -cocci in groups of eight. e.g., Sarcina ventriculi
Streptococci and diplococci mostly occur due to the non-separation of daughter cells after cell division.
Sarcinae are organised in cubical pockets as cell division alternates amid three perpendicular planes.
ii. Rod-shaped bacteria also called the bacilli.
iii. Spiral shaped bacteria also called the spirilla.
iv. Comma shaped bacteria also called the vibrios.
v. Corkscrew-shaped bacteria also called the spirochaetes.
ii. Gram-negative bacteria.
On the basis of their mode of respiration
i. Aerobic bacteria or Aerobe
ii. Anaerobic bacteria or Anaerobe
On the basis of their mode of nutrition
i. Autotrophic Bacteria
ii. Heterotrophic Bacteria